The present invention relates to centrifuges and particularly to decanter type centrifuges. The invention is specifically directed toward a centrifuge having a baffle or the like which acts as a dam to restrict solids passing towards discharge.
A decanter-type centrifuge comprises a rotating bowl, typically having a cylindrical portion and a frusto-conical end portion which are substantially imperforate. The rotation of the bowl creates a centrifugal force which separates a liquid feed mixture into its constituent parts. The feed mixture within the bowl forms a cylindrical pond, with a ring or layer of separated heavy material adjacent the inside of the bowl wall and a ring or layer of lighter material radially inward of the heavy material layer.
The terms xe2x80x9cheavy phasexe2x80x9d and xe2x80x9clight phasexe2x80x9d are sometimes employed to describe the materials which are separated from the feed mixture by the application of centrifugal force. In a decanter centrifuge having a conveyor, the light phase material will usually be a liquid and the heavy phase material will usually be a mixture of solids and liquid. The liquid feed mixture or slurry introduced into the bowl generally has a specific concentration of suspended solids or other insoluble materials therein. These solids are concentrated by the centrifugal force to form within the rotating bowl a heavy phase, including coarse solids, fine solids and liquid.
In a decanter centrifuge, a coaxially mounted screw conveyor rotates inside the bowl at a slightly different speed from the bowl. The flights of the screw conveyor push the separated heavy phase along the inside of the bowl wall towards the frusto-conical end of the bowl. Discharge ports for the separated heavy phase are located at the relatively smaller diameter end of the frusto-conical bowl portion. The separated light phase liquid is discharged by flowing from the cylindrical pond through separate discharge opening or ports. The light phase liquid discharge ports are located, typically, at the opposite end of the bowl from the heavy phase discharge.
Separation of the heavy phase materials from the feed mixture is a function of the residence time of the mixture in the bowl, the feed rate, the difference in specific gravity of the solids of the heavy phase from the liquid of the light phase, and the ability of the centrifuge to separately discharge the heavy and light phase materials. The purpose of the decanter centrifuge is to separately discharge a concentrated heavy phase and a clarified liquid. In order for the heavy phase to be discharged, it must be moved up the incline of the inside surface of the frusto-conical end of the bowl, called the beach, against the centrifugal force component acting in the opposite direction, downward along the beach (away from the heavy phase discharge).
One form of decanter centrifuge is shown in U.S. Pat. No. 3,795,361 to Lee. The centrifuge includes a conveyor having a central conveyor hub and a spiraled conveyor flight extending for at least a portion of the longitudinal length of the bowl. A feed pipe extends along the axis of the conveyor terminating in a feed chamber within the conveyor hub. A feed mixture is introduced into the feed chamber via the feed pipe. The feed engages accelerator vanes for imparting radial and tangential velocity thereto. Once the feed is brought up to speed, it is discharged out of the feed chamber through feed passages and into the centrifuge bowl where separation occurs. The centrifuge includes a baffle or partition mounted on the conveyor hub. In one embodiment, the baffle is a radial disc mounted adjacent the joint between the cylindrical and frusto-conical portions of the bowl. In another embodiment, the baffle is a conical structure mounted within the frusto-conical portion of the bowl. The baffle, whether conical or a disc, extends radially from the conveyor hub and penetrates into the heavy phase solids layer to form an annular hydraulic seal. In addition, the radial position of the light phase discharge is positioned inward of the position of the heavy phase discharge. This relationship, in conjunction with the penetration of the baffle into the heavy phase layer, creates a centrifugal pressure head that assists in pushing the solids up the beach towards discharge. Lee U.S. Pat. No. 3,795,361 is herein incorporated by reference.
Variations on the use of a baffle and centrifugal pressure head to assist in solids discharge are known. For example, U.S. Pat. No. 3,934,792 to High et al. shows an axial baffle extending between two adjacent portions of the conveyor flight. Also, U.S. Pat. No. 5,354,255 to Shapiro discloses a decanter centrifuge including a radial baffle positioned adjacent an open feed chamber formed within a discontinuity in the conveyor hub. A plurality of vanes extend across the feed chamber to create structural integrity for the conveyor. A ribbon-type conveyor flight is provided in the separation portion of the bowl.
In positioning a baffle on a conveyor hub, there is created a confined area where the baffle meets the spiral of the conveyor flight. The solids rotate with the bowl and, because of the differential rotational speed of the conveyor, are propelled axially by the conveyor flight toward the heavy phase discharge ports at one end of the bowl. This movement also causes the heavy phase to be pushed or wedged into the confined area between the flight and the baffle, creating a solids jam which may bend the baffle and the flight and which may cause the conveyor to be out of balance.
The present invention relates particularly to a decanter-type centrifuge of the type having a bowl rotatable about its longitudinal axis and a coaxially mounted screw conveyor rotating at a relative speed with respect to the bowl. The screw conveyor includes a spiraled conveyor flight extending radially outwardly to a position adjacent the inside wall of the bowl for conveying separated heavy phase material toward the heavy phase discharge in the bowl. A baffle extends from the conveyor hub and is adapted to project into the separated heavy phase material. A shelf is positioned between the baffle and the forward or conveying portion of the conveyor flight to close the restricted area between the baffle and the conveyor flight.
In one embodiment of the present invention, the shelf forms an included angle with any radially extending line that is greater than 45 degrees, and preferably in the range of 60 to 75 degrees.
In another embodiment of the present invention, the shelf is curved and has an outwardly facing convex surface. Preferably, the curved shelf forms an included angle with a radially extending line that is greater than 45 degrees, and preferably in the range of 60 to 75 degrees.
In a still further embodiment of the present invention, the shelf forms the baffle by extending between adjacent portions of the conveyor flight. The shelf baffle is preferably curved and has an included angle greater than 45 degrees.